Linksys WRT1900ACv2 (Cobra) 5GHz WiFi questions

Hi all,

I have some problem with my WRT1900ACv2 on 5GHz WiFi, flashed 21.02.2 and using my router to connect another home router (Asus RT-AX82U) 5GHz band to create a test network (since my old devices has ethernet + very old WiFi, using Linksys as WiFi client and convert to LAN can increase speed), I found that OpenWRT GUI telling me "connected" but DHCPCD is unable to get IP for WWAN, and I checked from my Asus router log the Linksys is actually disconnecting itself like every 40-50s (also confirmed Linksys has no IP assigned from Asus). To avoid problem I disabled AP mode (so Linksys is purely a WiFi client) but same issue, from both Linksys/Asus router log I can't see anything unusual.

I wiped the router to start over which doesn't change anything, until I put back 19.07.9r11405 and it works! I can see that DHCPCD is able to get IP from Asus router and connect to internet, so why is the 21.02.2 doesn't work?

2nd question is about NSS, both Linksys/Asus are 4T4R router, I was expecting my Linksys would connect to Asus with NSS = 4 which fully utilize 4T4R and get 1733Mbps link rate, instead it's doing 3T3R (Asus is reporting NSS = 3 only), is there any limitation when we use the router as WWAN/WDS bridge?